The invention concerns power tools with a rotary output and particularly tools having more than one mode of operation and an adjustable means for limiting the output torque in at least one of the modes of operation.
Torque limitation or control is desirable or necessary in several modes of rotary power tool operation. For example, in a powered screwdriver used for setting screws or running nuts, torque control is necessary for achieving the performance specifications of the fastener, or at least avoiding the stripping of threads. In a hammer drill, constant slipping of a ratcheting clutch at a predetermined substantially constant torque may produce the vibration which gives the tool its hammering effect. Typically, clutches used in rotary power tools depend for torque transmission on the biasing together of opposing toothed clutch faces with some axial engagement or overlap of the teeth, although plain friction discs are used in some applications.
In a common form of powered screwdriver, providing only the screwdriver mode of operation, final torque transmission into the output spindle depends upon engagement of a positive drive clutch responsive to partial axial retraction of the output spindle when pressure is applied to the workpiece. Torque limitation is provided by a second clutch in which opposing clutch halves are biased together by a compression spring. Disadvantages of this arrangement are the relative bulk and weight of the torque limiting clutch and inconvenience in adjusting the spring pressure. See, for example, West German Patent DE 2,325,235 Pfab and European Patent EP 0,178,252 Duerr.
An improved screwdriver arrangement (in a single purpose tool) is disclosed in U.S. Pat. No. 3,834,252, Abell, also assigned to the assignee of the present invention. Here, driving engagement of the output spindle again depends on axial displacement of the spindle responsive to work piece pressure, but a single clutch is used for both drive engagement and torque limitation. Adjustably precise control of axial depth of tooth engagement in mating clutch halves, combined with a clutch tooth profile of varying pressure angle provides, selectively, a wide range of torque limitation. Advantages of the design are its compactness and relatively lower cost and lighter weight and particularly the convenience and repeatable control of the torque limitation adjustment.
In combination tools, having two modes of operation (output modes), such as screwdriver/drills or hammer/drills, the tool transmission must be readily convertible from a condition providing a first output mode such as drilling (continuous rotation, usually without specific task torque limitation) to a second condition in which output torque is limited to suit the task in hand. Typically the same torque transmitting clutch is used for both modes. The clutch is mechanically maintained in firm engagement for the "live spindle" sustained rotation as in drilling. For a screwdriving or hammering mode the clutch is selectively engaged and torque limitation while in engagement may depend on operator provided bias of the tool against the workpiece. See for example British Patent 1,601,257 also assigned to the assignee of the present invention. With this arrangement, insofar as variable output torque limitation is available in a single tool, torque level and repeatability depends largely on operator skill in controlling his pressure of the tool against the workpiece.
In another form of combination screwdriver/drill, disclosed in U.S. Pat. No. 4,823,885, Okumura, a single spring loaded axial engagement type clutch is in constant engagement in both modes of operation, except when slipping at a predetermined torque in the screwdriver mode. A relatively convenient single adjustment arrangement permits an operator both to adjust clutch spring compression to provide torque limitation at a selected level for the screwdriver mode and to compress the spring fully for firm, non-slipping engagement of the clutch for the drill mode. An indicator in the adjustment system allows the operator to return at will to a previously used torque setting but, even so, it is inconvenient to be required to "lose" the torque limit setting each time the tool is shifted from one mode of operation to the other.
Clearly it would be desirable when using a combination tool for repetitive work that the torque limitation setting of one mode of operation remain undisturbed when the tool is shifted to a second mode of operation and again returned to the first. Obviously, in a simple transmission, the clutch used for a variable torque limitation mode must also be convertible to the "live spindle" (or no specific torque limitation) mode. Shifting from one operating mode to the other becomes essentially a matter of clutch differentiation or clutch control. Although desirable, there is an inherent conflict or difficulty in arranging for a single adjustable torque transmitting clutch, operable selectively in either one of two conditions, to be convertible to the second condition without disturbing the adjustment setting of the first condition. No such desirable arrangement is known.